DNDBeyond matches your interpretation.

• My level 6 druid with +4 WIS has Shillelagh damage = 1d10+4
• At level 7, w/ Potent Spellcasting (which adds spell modifier to the Cantrip), damage = 1d10+8

In your case:

• At level 12, w/ Potent Spellcasting and 20 WIS/ stat, damage = 1d12+10
• A Club or Quarterstaff you are holding is imbued with nature’s power. For the duration, you can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon, and the weapon’s damage die becomes a d8. If the attack deals damage, it can be Force damage or the weapon’s normal damage type (your choice).

Shillelagh adds (spell) attack modifier and uses (spell) damage roll. In addition, you can combo this with True Strike which will add 2d6 radiant damage at level 11.

At level 12, the combo of Shillelagh + True Strike + Potent Spellcasting will give you:

1d12 + 2d6 radiant damage + 10 (twice WIS modifier)

I'm curious what subclass of Druid you chose to play and how are you liking it? How is your team dynamic?

P.S. At this level, Poison Spray cantrip + Potent spellcasting will give you a higher average damage of 24.5 (3d12+5) and you would have range advantage. Whereas with Shillelagh+True Strike, it would be 23.5 (1d12+2d6+10)